The sort-by-juxt idiom was described on briancarper.net[1], where it
was refined for Clojure 1.1 by Malcolm Sparks. Andy Fingerhut has
also written about it, thoroughly.[2]

Such lore gives it the odor of respectability, but the sort-by-juxt
idiom takes liberties beyond the documented behavior ("contract") of
vectors. APersistentVector indulges the idiom, but the clojure.org
Data Structures page does not say how vectors should compare.

The vector specification should be bolstered with enough traits of
vectors' sorting behavior to make the sort-by-juxt idiom safe to use
wherever Clojure might be implemented.

"(sort-by (juxt" relies on a feature of vectors that the Clojure documentation does not guarantee. But using juxt in this way is part of the cultural fabric and helps make concise programs that "read like a definition" of the problem, to quote Halloway in "Programming Clojure". Therefore, let's document the sort order of equal-length vectors. I looked for this information on the Data Structures page, which has a section on vectors.